The invention relates to an auxiliary nose landing gear for installation on an aircraft.
The auxiliary nose landing gear according to the invention can be used on all aircraft types and in particular on civil and military aircraft for transporting passengers and goods.
The landing gear of an aircraft is designed for withstanding the different loads transmitted between the aircraft and the ground and in order to permit the taxiing of the aircraft on landing, takeoff and during movements on the ground.
The loads which the landing gear have to withstand vary as a function of the aircraft loading conditions, the speed, possible braking and the trajectory (straight line or turning). Moreover, as a result of these different conditions there is a variation in the load distribution between the auxiliary nose landing gear (called xe2x80x9cnose gearxe2x80x9d throughout the remainder of the text) and the main landing gear (called xe2x80x9cmain gearxe2x80x9d in the remainder of the text). Thus, when the aircraft is stationary, the static loads withstood by the nose gear are low (5 to 20% of the aircraft weight as a function of its geometry) compared with those withstood by the main gear. However, in certain dynamic phases, the loads withstood by the nose gear can become significant (sometimes roughly double the static loads). The nose gear then has to withstand a high compressive stress.
The standards imposed by the certification authorities strictly control the behaviour of each of the elements of an aircraft landing gear under both normal and exceptional conditions.
Thus, in the case of dynamic braking, the nose gear must be able to take up 30% of the load exerted in accordance with the vertical axis. When the aircraft is towed, the stress withstood by the nose gear is directed along the longitudinal axis of the aircraft. In the case of a crabwise landing, the nose gear must withstand a stress exerted in accordance with the lateral or transverse axis of the aircraft and whose level reaches 0.8 times the load withstood in accordance with the vertical axis. The nose gear must also withstand a high torque when oriented when the aircraft is stationary. The nose gear must also be dimensioned in such a way as to take account of the critical case of an aborted takeoff, which corresponds to an emergency braking when the aircraft is at its maximum weight and slightly below its takeoff speed. When this case occurs, the main gear sheds 30% of its load, which is transferred to the nose gear.
In addition, the loads transferred to the runway by the wheels of the landing gear must not exceed the maximum load threshold of said runway, which is in particular dependent on the surface used, the nature of the subsoil, etc.
Finally, the nose gear can be oriented in order to ensure the guidance of the aircraft during taxiing on the runway and when the aircraft speed is inadequate for guidance by the vertical rudder. In the case of a tyre burst, the aircraft may prove very difficult to guide.
In practice, designers have increased the number of wheels equipping the nose gear of aircraft in order to respect these different constraints and ensure a completely satisfactory behaviour of the aircraft on the ground. Thus, the nose gear of aircraft, whose weight on takeoff is below 250 tonnes is generally equipped with two wheels, whereas with aircraft having a takeoff weight exceeding 250 tones, they are normally equipped with nose gears having four wheels.
In other words, the nose gear of aircraft is oversized to take account of constraints only appearing during very short aircraft use phases. This oversizing of the nose gear more particularly relates to the number of wheels.
In this connection it should be noted that the cost resulting from changing worn landing gear tyres increases with the number of wheels equipping the nose gear. The nose gear strut must also be oversized to take account of the torsional moments produced by the skidding of tyres on the runway when turning.
The invention specifically relates to an aircraft nose gear, whose original design makes it possible for it to only use a limited number of wheels under normal landing, takeoff and taxiing conditions, so that the dimensioning of the gear strut and the cost of changing tyres can be reduced compared with the nose gears which at present equip aircraft.
According to the invention, this result is obtained by means of an auxiliary nose landing gear for the aircraft, which comprises a main group of at least one wheel which can be in permanent contact with the ground when the aircraft is on the ground, said gear being characterized in that it also comprises a secondary group of at least one wheel, which can normally be spaced from the ground when the aircraft is on the ground, when the loads transmitted through the gear do not exceed a predetermined threshold and the main group is operational.
In other words, the nose gear according to the invention comprises a main group of at least one wheel in permanent contact with the runway when the aircraft is on the ground and a secondary group of at least one wheel only coming into contact with the runway and only withstanding loads under critical operating conditions such as emergency braking resulting from an aborted takeoff or under abnormal conditions such as e.g. the bursting of a tyre of a wheel of the main group.
Under normal conditions, only the wheel or wheels of the main group are consequently used and become worn. However, the presence of the wheel or wheels of the secondary group make it possible to maintain the control of the trajectory of the aircraft, particularly in the case of a tyre bursting and to uniformly distribute the loads over the runway when they increase beyond a certain threshold.
In a preferred embodiment of the invention, the nose gear comprises a gear leg having a substantially vertical longitudinal axis when the aircraft is on the ground, each wheel of the main group being installed at the lower end of the gear leg and each wheel of the secondary group being installed on at least one articulated arm at the lower end of the gear leg. Actuating means are then interposed between the arm and the gear leg in order to pivot the arm between an extended position and a retracted position.
In order to lock each wheel of the secondary group in a position such that it remains in contact with the ground when it is active, abutment means are advantageously provided to serve as a support for the arm when in the extended position.
When the aircraft is on the ground and with the arm in the extended position, each wheel of the secondary group can be displaced either forwards or rearwards with respect to each wheel of the main group.
In a comparable manner, when the landing gear is retracted and when the arm is in the retracted position, each wheel of the secondary group can be displaced forwards or rearwards with respect to each wheel of the main group.
To ensure a better support on the ground, the axle of each wheel of the main group is advantageously offset slightly forwards or rearwards with respect to the longitudinal axis of the gear leg when the aircraft is on the ground.
In the preferred embodiment of the invention each wheel of the secondary group has a diameter smaller than that of each wheel of the main group. This arrangement leads to a considerable space and weight gain.